1. Field of the Invention
The present invention relates to an electric power controller for vehicle mounting, and particularly relates to a device for controlling electric power supply to load of large consumed electric power, especially, an electric power controller for vehicle mounting having a preventing function of a battery rise.
2. Description of the Related Art
In various electric mounting products mounted to a vehicle, considerably large electric power is consumed in e.g., an audio system of large output and a front illuminating light of a xenon system, etc. Therefore, a burden on the battery is large. Further, a commercial electric power converter (a machine for converting a battery voltage into commercial electric power of AC100V) able to use a domestic appliance such as television, etc. is also arranged in accordance with vehicles. The battery burden is also large when such a domestic appliance is used.
Therefore, when the electric mounting product and the electric product of large electric power consumption (these are generically called load in this specification) are used, for example, the battery voltage is monitored as shown in the following patent document 1. When this battery voltage is smaller than a predetermined reference voltage, electric power supply to the load is interrupted and the battery rise is prevented.
FIG. 4 is a view showing a conventional electric power controller for vehicle mounting. In this figure, a battery 1 can be charged by a dynamo 3 operated by an engine 2. A terminal voltage (hereinafter called battery voltage Vb) of the battery 1 is supplied to load 6 through a harness 4 and an electric power controller 5.
The electric power controller 5 interrupts an electric current i when an input voltage is smaller than a predetermined judging reference voltage (hereinafter called a threshold value SL). For example, the load 6 is an audio system of large output, a front illuminating light of a xenon system, etc. or a domestic appliance such as television, etc. in accordance with the above illustration. However, when the load 6 is set to the domestic appliance, the electric power controller 5 is also used as a commercial electric power converter. Namely, in this case, in addition to the above electric current interrupting function, the electric power controller 5 can convert voltage Vb_in of a load side terminal of the harness 4 into commercial electric power of AC100V and supply this commercial electric power to the load 6.
Here, when a lowest voltage of the battery 1 required to start the engine 2 is set to be called a protecting voltage Vh, the electric current i to the load 6 is interrupted to protect the battery 1 when the battery voltage Vb is smaller than the protecting voltage Vh. Accordingly, it seems that it is sufficient to set the threshold value SL of the electric power controller 5 to the same value as the protecting voltage Vh. However, in reality, the threshold value SL must be set to a value lower by a certain voltage than the protecting voltage Vh. This is because an input voltage of the electric power controller 5 is voltage Vb_in of the load side terminal of the harness 4, and this voltage Vb_in is lower than the battery voltage Vb by a voltage given by a product (ixR) of the electric current (electric current i flowed to the load 6) flowed to the harness 4 and a resistance value R of the harness 4. The voltage given by this ixR is next called a correction voltage Ref.
For example, in the prior art described in the following patent document 1, when an electric power voltage (corresponding to the above voltage Vb_in) of a direct current power source (corresponding to the above battery 1) becomes lower than 6 V (corresponding to the above predetermined voltage SL), electric current supply to a high brightness discharge light (corresponding to the above load 6) is interrupted and this discharge light is turned off (see paragraph number [0041], etc. of the same document). In accordance with the above idea, the threshold value SL (=6V) in such a prior art is SL=Vh−Ref. Accordingly, for example, if i=10A and R=0.2Ω, Ref=ixR=2V is formed and 6V=Vh−2V, therefore, Vh=8V is formed. Accordingly, when the battery voltage Vb is smaller than 8V, the electric current i to the load 6 can be interrupted and the battery rise can be prevented.
[Patent document 1] JP-A-2004-273172
However, in the former electric power controller for vehicle mounting, there are the following problems since the threshold value SL is set to a fixed value set in advance.
As mentioned above, SL=Vh−Ref, and Vh is a tentative standard of battery protection so that Vh may be a fixed value. However, fixed Ref causes incongruence for the following reasons. Since Ref is given by the product (ixR) of the electric current i of the load 6 and the resistance value R of the harness 4, the actual ixR and the fixed Ref are not conformed when the load 6 is changed and the harness 4 is changed.
A first case of the unconformity is a case in which “Ref>actual ixR” is formed. In this case, if the difference between both the voltages is set to α, i.e., if Ref=(ixR)+α is set, the electric current i is interrupted when the battery voltage Vb is smaller than “protecting voltage Vh+voltage α”. Therefore, the trouble that protecting ability of the battery is reduced by the voltage α and the battery rise is easily generated is finally caused.
On the other hand, a second case of the above unconformity is a case in which “Ref<actual ixR” is formed reversely to the above case. In this case, if the difference between both the voltages is α, i.e., if Ref=(ixR)−α is set, the electric current i is interrupted when the battery voltage Vb is smaller than “protecting voltage Vh−voltage α”. Accordingly, in this case, a protecting function of the battery 1 is early fulfilled by the voltage α. Therefore, the trouble that performance (charging state) of the battery 1 is sufficient, but no load 6 can be used is caused.